This invention relates to a novel method of casting a dissolvable ophthalmic shield. The novel method provides enhanced control of the solubility, comfort and clarity characteristics of the resulting ophthalmic shield.
Methods for preparing collagen soft contact lenses are well-known, and they vary in the specific steps taken to produce the lens. In general, the first step is to extract the collagen from animal hide, such as steer hide or pigskin, by dissolving the hide with acids, bases, salts or by enzyme extraction. The collagen also can be recovered in fiber form, in which the collagen is not dissolved but is recovered from an aqueous dispersion.
If a method in which the collagen is dissolved is utilized, the solution is centrifuged to separate the collagen gel and remove any unwanted debris. The collagen gel then can be placed in a mold where it is centrifuged to make the collagen gel spread evenly across the mold surface to form a film. Alternatively, the collagen gel can be centrifuged to remove unwanted bubbles before being placed into a mold to be fixed or cross-linked.
The fixing process or cross-linking of the collagen gel stabilizes the collagen to prevent it from dissolving. Cross-linking can be carried out by irradiation with gamma or ultraviolet rays or by chemical means or by a combination of both. In conventional methods, the cross-linking process is carried out while the collagen gel is in the mold. The collagen gel can be wet or dry at the time of cross-linking. Once the cross-linking process has been completed, the finished lens is clear, flexible and biostable (non-dissolvable in physiological fluids). See, for example, U.S. Pat. Nos. 4,223,984; 4,264,155; 4,505,855 and 4,581,030. Fixing or cross-linking also transforms the collagen gel into a flexible, plastic film which has the dimensional stability necessary to allow it to be removed from the mold. The physical strength, tear-resistance and biostability of the resulting collagen shield increases with increased cross-linking.
In addition to its use in forming biostable contact lenses, collagen can be used to form a soluble optical insert which acts as a sustained delivery device for a pharmaceutically active agent. The optical insert can be placed in the eye, such as in the inferior fornix or the inferior cul de sac, where it slowly dissolves under physiological conditions, releasing the drug.
Cross-linked collagen generally must be chemically modified before it becomes significantly soluble under physiological conditions. Such chemical modification includes, for example, acylation of the amino groups of the collagen with acetic anhydride or other anhydrides, such as succinic anhydride, or esterification of the carboxyl groups with acidified alcohol such as methanol or ethanol. See, for example, U.S. Pat. No. 4,164,559, relating generally to chemical modification of collagen.
Currently, for drug delivery to the eye, the main dosage forms are drops and ointments. For both forms, the duration of the therapeutic effect is short, and repeated doses are required during the course of therapy because the drugs are rapidly washed out by reflex tear flow. In addition, strict patient compliance is required to achieve the desired therapeutic effect.
Although there exists a method for preparing a biosoluble ophthalmic membrane containing an ophthalmic drug, which can be inserted into the eye, the ophthalmic membrane is not a shield that can be comfortably worn on the eye. Further, current methods where collagen is used to make the soft contact lenses lead to the formation of a biostable lens. Therefore, a need exists for a method of preparing an ophthalmic membrane with defined geometries that can optionally contain an ophthalmic drug that will dissolve under physiological conditions while being comfortably worn on the eye.